The Apple Watch running watchOS3 beta 1 did an incredible job of measuring my heart rate today while I golfed in the heat and humiditywo h. In the past, I have had good success with the heart rate monitoring, but when golfing, I always lost the heart rate 5-10 times and sometimes it took minutes to come back. It never bothered me, but when i looked, it would say 54 or "measuring" several times. In the end, I would have a decent report of my heart rate with several dips to 54. Whenever it said "measuring" I think I read that it will use the last known rate.
Anyway, with watchSO3 beta 1, I never lost my heart rate for the two hours that I golfed. It was 85 and humid and I must have checked the workout screen 100 times and it was always measuring my heart rate. I could tell it was spot on as well as I know how hot it was and as I walked and watched, it would climb and then back off when I was waiting to hit.
Two hours without one lost of the heart rate might be a good sign for those who have said the heart rate monitoring wasn't working for them.
Also, the workout app looks very nice with new fonts and colors - when I was up to 1:45 minutes, I noticed the seconds were being cutoff so I could see 1:45:3 and a missing number.

Very impressed with the workout app layout and font and colors for heart rate.
Hope others have success with the heart rate monitoring.